By default people can't access the Steam overlay while using the GameRanger application and many people have complained about it. I figured out a simple fix to make that happen. All you have to do is add GameRanger.exe as a non-Steam game. Then in your Steam library, run GameRanger, then pick a SWBF2 server and join. You should be able to use the Steam overlay now.

GameRanger.exe is located in C:\Users\Anthony\AppData\Roaming\GameRanger\GameRanger (Anthony = your PC name)

Technically this should work with any game, I just have only tested with SWBF2.